It is well known that lubricants such as oil are commonly used to minimize wear in operating machine parts such as transmissions, gearboxes, hydraulic systems and engines used in aircraft, helicopters, ships, locomotives, trucks, automobiles, stationary engines and pumps as examples. However, in spite of lubrication, particles wear from rubbing and bearing surfaces and deposit in the lubricant. It has long been recognized that knowledge about the amount and form of these wear particles can give valuable information concerning the condition of the machine. In particular, it has been recognized that the presence of metallic wear particles is an indication of the presence of an abnormal wear condition. In many cases metallic particles larger than 10 microns are produced by said abnormal wear condition. When said abnormal wear condition is starting said particles are present in very small amounts, even less than 10 micrograms per cubic centimeter. Therefore sensitive techniques for detecting said particles are needed in order to give early warning of said abnormal wear condition.
Since the highly stressed rubbing and bearing surfaces in modern machines are usually made of cast iron or steel the wear particles from these surfaces contain iron.
Many techniques have been used to analyze for iron containing wear particles in used lubricants. One current method is to periodically sample the lubricant and submit it for laboratory analysis by spectroscopy. However, spectroscopy does not distinguish metallic iron and steel from other forms of iron such as oxides and corrosion products not associated with said abnormal wear condition. In addition, it is insensitive to particles larger than about 8 microns. Furthermore, it requires an expensive laboratory instrument and is, consequently, not available to many machine operators.
Since metallic iron and steel are strongly magnetic, special magnetic techniques have been used to detect metallic iron and steel wear particles. In one such technique the particles are magnetically deposited on a glass slide. After suitable washing and fixing, the amount deposited is determined from the optical density of the deposit. However, other materials, less magnetic than metallic iron and steel, such as carbonaceous combustion products, basic sulfates, oxides, corrosion products and the like, also deposit and contribute to the optical density. Consequently optical density is a poor measure of the amount of metallic iron and steel wear particles.
Some machines contain magnetic chip detectors in their lubricant reservoirs that collect iron and steel wear particles from the passing circulating lubricant. However, these detectors are insensitive to particles smaller than about 100 microns and do not provide a quantitative measure of the amount of metallic iron and steel wear particles in the lubricant. They are, of course, only useful for machines in which they are already installed. A method for measuring the quantity of magnetic particles in a fluid has been described in Japanese Patent Application Kokai No. 59-54956(54956/1984). This method is characterized by a magnetic collecting part to collect the magnetic particles and a magnetic flux sensor to measure the quantity collected. However, the method as described, while quite suitable for measuring the performance of electromagnetic filters, does not have sufficient sensitivity and accuracy for measuring the amount of iron and steel wear particles in a sample of lubricant from a machine for the purpose of giving early warning of an abnormal wear condition.